A long as it isn't WAY loud, you'll be ok.

Is it A Hd. emissions  8600 GVWR truck
If your 3/4 ton K-20 is 6000 GVWR you will need A cat and all that HP
robbing stuff
I know that my 1981 k-20 didn't have cats with Hd. emissions 8600 Gvwr
all it has was the AIR injection system but it fell off just before the
heeder were installed!
If you don't need A cat just get A Flowmaster muffler
you can't beat the flowmaster for performance and sound
http://www.flowmastermufflers.com/

     California law requires that all vehicles must be equipped with an
adequate muffler to prevent excessive noise from the exhaust system. It also
prohibits the operation of a passenger vehicle (other than a motorcycle), or
a truck with a Gross Vehicle Weight Rating (GVWR) of less than 6,000 pounds,
that has an exhaust noise level greater than 95 decibels, when tested under
specified conditions.
     If you have been issued a citation for operating a vehicle in
violation of Vehicle Code Section 27150 or 27151, you must take your vehicle
to a state Referee Center.

     Note: The Referee is only authorized to inspect and certify passenger
cars and trucks under 6,000 GVWR that have been cited for a violation of
Section 27150 or 27151 of the Vehicle Code. The maximum decibel level
applies only to those vehicles. Citations issued to other vehicles
(motorcycles, trucks exceeding 5999 pounds GVWR) are not part of this
program. Check with the agency that issued the citation, and/or the court,
to find out how to show proof of correction for these other vehicles.

     The following steps outline the procedures you'll need to follow to
get a certificate of compliance, which shows your vehicle's exhaust noise
level is within the applicable standards.

       STEP 1

         If you feel your vehicle's exhaust system has been modified or has
deteriorated so that the noise level exceeds current standards, you should
have the exhaust system repaired before proceeding to step 2.

         OR

         If you have reason to believe that your vehicle's exhaust noise
level meets the current standards, proceed directly to step 2.

         [Note that citations have a court appearance date.]
         [Timely action can help you avoid additional fines and penalties.]

       STEP 2

         Have your vehicle tested at a state Referee Center. To make an
appointment, call the Referee Scheduling Center at (800) 622-7733. Have the
citation and vehicle registration with you when you call. The Center will
tell you any fees that will need to be paid for the test.

       STEP 3

         The Referee will conduct an exhaust noise level test in accordance
with the Society of Automotive Engineers (SAE) standard J1169, May 1998, to
determine if your vehicle's exhaust noise level exceeds the noise standard
(95db).

         If your vehicle meets the standard, the Referee will give you a
"Certificate of Compliance." Take the certificate to the court (See Step 5).

         If your vehicle does not meet the standard, the Referee will give
you a report that lists the recorded decibel readings. Your vehicle must be
repaired to meet the standard of not more than 95 decibels (See Step 4).

       STEP 4

         If your vehicle does not meet the standard, additional repairs
must be made before you can make a second appointment with the Referee
Scheduling Center.

       STEP 5

         Once your vehicle receives a "Certificate of Compliance" from a
Referee Center, present the certificate to the court as proof that your
vehicle is in compliance with California law.

         NOTE: Inspection fees and any necessary repairs are your
responsibility. You may also be required to pay a fine and other legal costs
when reporting to the court. Also, note that most citations have a deadline.
Timely repairs can help you avoid additional fines and penalties.

     If you have any questions, please contact the local law enforcement
agency that issued the citation or the Bureau of Automotive Repair at (800)
952-5210.

     See link below

     http://www.smogcheck.ca.gov/stdPAge.asp?Menu=/includes/Menu_GenInfo.htm&Body=/Ge
ninfo/Factsheets/Vehicle_Exhaust_Noise_Level_Certification-Sep_2003.htm

     Dave

     Las Vegas NV.

     September 2003